The vast majority of hearing and balance impairments are thought to be due to death of sensory hair cells, the receptor cells of the inner ear. These cells are unusually metabolically active and hypersensitive to damage from overstimulation, some therapeutic drugs and environmental toxins, and aging. However, there is enormous variability in structural and functional outcomes of these challenges to the inner ear among both humans and laboratory animals, and a large number of mutations either directly influence the viability of hair cells, or alter susceptibility to ototoxicity. The goals of the research program proposed herein are: A). to better define the cellular and molecular cascades that control hair cell death and survival following exposure to potentially ototoxic agents; and B). to use the genetic potential of the zebrafish and the accessibility of the lateral line neuromasts to identify and characterize genes that influence the viability of hair cells when challenged by ototoxic agents. The proposed experiments profit from extensive pilot work carried out over the past year and the combined expertise of two laboratories. One laboratory has extensive experience studying zebrafish developmental neurobiology, including development of the lateral line system, while the other has been studying development, regeneration and degeneration in the auditory system of birds and mammals for the past 3 decades. Four groups of experiments are proposed: 1). We will compare the structural events and molecular cascades that occur in lateral line hair cells following neomycin ototoxicity with those reported in the inner ear of birds and mammals. 2). We will study the detailed sequence of cellular events that lead to hair cell death or survival, correlating time lapse imaging with electron microscopy of cellular ultrastructure. 3). We will characterize two dominant mutations that influence hair cell susceptibility to neomycin toxicity, and propose to map the chromosomal location of these genes. 4). We propose to initiate two new screens for modifier genes that alter hair cell response to ototoxic agents. In addition to the goals stated above, these experiments will assess the general feasibility of interaction screens using zebrafish.